Ultrasound diagnostic apparatuses have been used for diagnosing many body parts related to cardiology, obstetrics and gynecology, etc., because they are noninvasive and free from radiation exposure. Recently, an IMT (Intima-Media Thickness) has received attention as an indicator of arteriosclerosis in a cervical blood vessel (carotid). As shown in FIG. 31, a cervical blood vessel 111 has a configuration in which blood 114 flows inside blood vessel walls 112, 113. Each of the blood vessel walls 112, 113 is composed of three layers of a tunica intima 115, a tunica media 116 and a tunica adventitia 117 arranged in this order from the inside. The IMT is a thickness of an intima-media complex of the blood vessel, which is obtained by converting time widths of ultrasonic echo parts of the tunica intima 115 and the tunica media 116 into distances.
FIG. 32 is a waveform diagram showing a waveform of a scan line echo signal detected from echoes of ultrasonic beams irradiated to the blood vessel. An ultrasonic probe receives echoes in the following order: a strong echo from the front wall 112, a weak echo from the blood 114, a slightly strong echo from the tunica intima 115 of the back wall 113, a weak echo from the tunica media 116 and a strong echo from the tunica adventitia 117. The IMT is proportional to time of receiving the slightly strong echo from the tunica intima 115 and the weak echo from the tunica media 116.
In the IMT measurement in conventional ultrasound diagnostic apparatuses, the array direction of a one-dimensional array probe (generally, a linear array) is matched with a longitudinal direction of the blood vessel, and an ultrasonic image such as one shown in FIG. 31 is obtained (for example, see Patent Document 1). As shown in FIG. 33, a first conventional ultrasound diagnostic apparatus 101a is composed of a probe 102, a transmission/reception unit 103, a long-axis scan data storage memory 104, a signal analysis unit 105, an IMT measurement unit 106, a scan converter 107, a display unit 108 and a control unit 109. The transmission/reception unit 103 causes the long-axis scan data storage memory 104 to store long-axis cross section data obtained by the probe 102. The signal analysis unit 105 detects data necessary for the IMT measurement among the data stored in the long-axis scan data storage memory 104. The IMT measurement unit 106 measures the IMT. The scan converter 107 combines a B-mode cross-sectional image and an IMT value, and displays the combined image on the display unit 108.
As the computation of the IMT, other than a computation using scan line echo signals themselves, there has been known a computation using an envelope shown in Patent Document 1, for example. FIG. 34 is a block diagram of a second conventional ultrasound diagnostic apparatus 101b that measures the IMT by envelope demodulation. As compared with the first conventional example, the ultrasound diagnostic apparatus 101b is provided further with a demodulator 110 that performs envelope demodulation on received scan line echo signals. FIG. 35 is a waveform diagram showing a demodulation signal obtained at the demodulator 110. The IMT measurement unit 106 computes a thickness of the intima-media part by measuring time corresponding to the slightly strong echo from the tunica intima 115 and time corresponding to the weak echo from the tunica media 116.